1) Field of the Invention
The present invention relates to an image forming apparatus having an electrostatic recording unit, and more particularly to a multicolor image forming apparatus performing multicolor recording by arranging a plurality of electrostatic recording units in series.
2) Description of the Related Art
In general, in an electrostatic recording unit utilizing the electrostatic recording technique, an electrostatic latent image is written on an electrostatic latent image carrying body such as a photosensitive body, dielectric body, etc.; the electrostatic latent image is electrostatically developed as a charged toner image by a developer, i.e., a so-called toner; the charged toner image is electrostatically transferred to a recording medium such as a recording sheet of paper; and then the transferred toner image is fixed on the recording medium by heat, pressure, light, etc.
As a multicolor image forming apparatus utilizing the electrostatic recording technique as mentioned above, a single-drum type of multicolor recording apparatus is known. This multicolor recording apparatus comprises a single electrostatic image carrying body, i.e., a photosensitive drum, and a plurality of developing devices using different developers composed of different colors, respectively, and arranged between an electrostatic image writing location, at which an electrostatic latent image is written on the photosensitive drum, and a transfer charger. For example, a multicolor recording apparatus for a full-color recording is provided with four developing devices, which use a yellow developer composed of yellow toner, a cyan developer composed of cyan toner, a magenta developer composed of magenta toner, and a black developer composed of black toner, respectively. An electrostatic latent image is first formed on the photosensitive drum on the basis of yellow image data, and is developed as a yellow toner image by the yellow toner. Then, the yellow toner image is transferred from the photosensitive drum to a recording sheet of paper, and is fixed on the sheet of paper. Subsequently, an electrostatic latent image is formed on the photosensitive drum on the basis of cyan image data, and is developed as a cyan toner image by the cyan toner, and then the cyan toner image is transferred to and fixed on the sheet of paper carrying the yellow toner image previously fixed thereto. A similar process is carried out also for the magenta image data and the black image data. Thus, the toner images of four colors are superimposed on the sheet of paper, whereby an image recording of full color is achieved. The single-drum type of multicolor recording apparatus has an advantage that the whole structure thereof can be made relatively compact, but toner images having different colors must be successively formed by a single photosensitive drum, and therefore there arises a problem in that high speed recording cannot be achieved.
Also, as a multicolor recording apparatus utilizing electrostatic recording techniques, a multi-drum type of multicolor recording apparatus is known, which comprises four electrostatic recording units aligned with each other along path for moving a recording sheet of paper. The respective electrostatic recording units includes developing devices which use a developer composed of yellow toner, a developer composed of cyan toner, a developer composed of magenta toner, and a developer composed of black toner. While a recording sheet of paper is moved along the path, a yellow toner image, a cyan toner image, a magenta toner image and a black toner image are sequentially formed on the sheet of paper by the four electrostatic recording devices, whereby a full-color image is obtained on the sheet of paper.
Although the multi-drum type multicolor recording apparatus as mentioned above is advantageous in that a high-speed multicolor recording can be carried out, it has a major disadvantage in that the construction thereof is large due to the alignment of the electrostatic recording units in series. Also, the apparatus is provided with a paper feeder unit for holding a stack of paper sheets to be recorded, and a paper receiver unit for accommodating a stack of recorded paper sheets, and these units are usually assembled in a housing of the apparatus so as to extend therefrom. Accordingly, the area occupied by the whole structure of the apparatus, for the installation thereof, that is, an area occupied by a projection of the whole structure of the apparatus onto the floor becomes larger. Further, since various constituent elements such as an electrical power source unit, an electronic controller unit, etc. are provided within the housing of the apparatus, it is required to reduce the overall structure of the apparatus in size by arranging these constituent elements in a compact size as a whole. Particularly, where the apparatus is constituted such that two-sided recording is performed on a recording sheet of paper, a paper bypass passageway must be provided for returning the sheet of paper, on which one-sided recording is made, to the path along which the electrostatic recording units are aligned in series, for the two-sided recording. In this case, the paper bypass passageway should be arranged in the apparatus housing in a compact size together with the above-mentioned constituent elements. In addition, in the multi-color image forming apparatus constituted in a compact size, ease of maintenance is liable to be sacrificed due to the compact size thereof, and therefore in the multi-color image forming apparatus, a reduction of size considering the ease of maintenance thereof is demanded.